The standards set forth by the Synchronous Optical Network (SONET) define optical carrier (OC) levels and the electrically equivalent synchronous transport signals (STS) for the fiber-optic based transmission hierarchy. For example, an OC-1 is equivalent to an STS-1 and provides a data transmission rate of 51.84 Mega bits per second (Mbps). Higher line rates are integer multiples of the base rate of 51.84 Mbps. In other words, an OC-3, and the corresponding STS-3, has a data transmission rate equal to 3 times 51.84 Mbps or 155.52 Mbps. Under the SONET standard, OC-3, OC-12, OC-48, and OC-192 are the most widely supported fiber-optic rates. However, other rates exist, e.g., OC-9, OC-18, OC-24, and OC-36.
As such, in a telecommunication network, there can be numerous types of connections that are established to handle signal traffic at the different transmission rates. These connections can include any combination of OC-1 connections, OC-3 connections, OC-12 connections, OC-48 connections, and/or OC-192 connections. In order to efficiently handle the signal traffic, it is often necessary to groom the signal traffic traveling over the network. Grooming involves rearranging and repacking low-speed demand, e.g., DS-1 demand, into higher speed connections, e.g., STS-1 connections, to obtain high utilization or fill ratios. Grooming allows demand from various destinations to be combined over a single transport connection.
In SONET ring based networks there are two ways to groom demand: centralized ring bandwidth management and distributed ring bandwidth management. In centralized ring bandwidth management, also known as “full hubbing,” all DS-1 demand originating from a node is packed into one or more STS-1 demands and transported to a central hub node. At the central hub node, the incoming STS-1 demands are dropped from the ring and connected to a wideband digital cross-connect system (WDCS). Within the WDCS, the individual DS-1 demands are cross-connected to groomed outgoing STS-1 demands, which are added back to the ring. Then, a number of co-destined demands can be transported to their common final destination.
An alternative to hubbing is distributed bandwidth management. In distributed bandwidth management, DS-1 demands are routed directly within the ring over a shared or collector STS-1. A collector STS-1 is an STS-1 time slot that is accessible by more than one pair of nodes. In order to access a collector STS-1, however, an add drop multiplexer (ADM) must be capable of time slot assignment (TSA) at the VT-1.5 virtual tributary level.
In a multi-level network, i.e., a network including an electrical level and one or more optical levels, the concept of traffic grooming, can be used when there is a significant volume of low-speed traffic between two nodes, e.g., central offices (COs). The demand in the multi-level network can be groomed to share the resources in the electrical and optical levels, thereby reducing the cost of the network. However, if each level is analyzed independently, it is possible that saving costs in one level, e.g., an optical level, can have a negative impact on the costs associated with the other level, e.g., the electrical level, and vice-versa. On top of this, if a SONET network is already installed in the field, it is necessary to make use of the SONET network in order to provide a smooth transition from the installed SONET network to a wavelength division multiplexing (WDM) based full optical network.